This invention relates to semi-trailer trucks and, more particularly, to over-the-road semi-trailer trucks for hauling heavier loads.
To ensure proper load distribution for safety reasons and to minimize road damage, government regulations establish maximum weight per axle for over-the-road semi-trailer trucks and require minimum spacing between certain axles. For example, present federal regulations require that single axles bearing up to a maximum of 20,000 pounds must be at least ten feet apart. For semi-trailer trucks rated for a maximum gross load of 80,000 pounds, both the tractor and semi-trailer usually have tandem axles spaced about four feet apart. The maximum gross weight imposed on the front steering axle of the trailer is 13,000 and 20,000 for any other axle. The normal weight distribution for a gross load of 80,000 pounds is 12,000 pounds on the tractor front axle, 34,000 pounds on the tractor tandem axles and 34,000 pounds on the semi-trailer tandem axles.
The wheels on the semi-trailer tandem axles are not steerable and the turning axis of the semi-trailer is at the midpoint of one axle or at a location between the axles. Consequently, when a corner or curve is being negotiated, the wheels on the or both axles are dragged sideways, causing the tires to be scuffed. This also imposes a torque on the semi-trailer support frame members which require these members to be beefed up and can cause premature fatigue. If the semi-trailer axles are spaced at least ten feet apart so that each individual axle can bear a maximum of 20,000 pounds, this problem is compounded.
Because of the relatively long distance between the tractor fifth wheel and the turning axis of the semi-trailer, the tractor must be swung wide when turning corners on the city streets and the like. This turning radius for longer semi-trailers can be great enough in some cases to make it impossible to drive the truck in closely confined areas.
When a semi-trailer truck is being dead-headed or hauling a light load, the load distribution may be such that government regulations could be met with a semi-trailer having a single load bearing axle in contact with the road. Thus, the tires on the extra axle and are not needed and worn unnecessarily. Also, the rolling friction from the unneeded axle reduces gas mileage.
Because of the relatively long span between the tractor fifth wheel and the semi-trailer rear axle, the semi-trailer support frame members must be quite heavy in order to provide the structural integrity required to support the load over that span. Substantially lower strength, and therefore, lighter frame members could be used if one of the two rear axles could be moved forward to reduce the bridge or span between the load bearing axles and the torque problem mentioned above could be minimized. Such a weight reduction also would reduce gas mileage and decrease tire wear.
Elimination of one of the tractor axles also is desirable. It can reduce the cost of manufacture and the cost of operation because of a reduction in gross weight and rolling tire friction, fewer tires and the elimination of tire scuffing caused by tandem axles. Also, a smaller horsepower engine could be used and still deliver the same horsepower to the ground.